Rabeprazole sodium, namely 2-[[[4-(3-methoxypropoxy)-3-methyl-2-pyridinyl]-methyl]sulfinyl]-1H-benzylimidazole sodium salt, of formula

is a gastric secretion inhibitor used for the treatment of peptic ulcer.
EP 268956 discloses the preparation of rabeprazole sodium by crystallization from ethyl ether, to obtain white crystals, having m.p. 140-141° C. (dec.). To date, six crystalline forms of rabeprazole sodium have been described, namely the one referred to as form II in JP 2001-39975, which is obtained starting from solid, non-crystalloid rabeprazole sodium; those referred to as forms X and Y in WO 03/082858; and that referred to as form Z in US patent application 2004/0180935. WO 03/082858 also mentions and claims the hydrated forms of the X and Y forms, but it neither discloses nor characterizes the preparation thereof. Finally, EP 1674463 discloses rabeprazole sodium in the crystalline hydrate form, in particular the two isomorphic forms alpha (substantially hemihydrate) and beta (substantially sesquihydrate). Different forms of biologically active compounds, in particular the polymorphic forms, are known to be useful both in therapy, thanks to their different bioavailabilities, release times and solubilities, and in the pharmaceutical technique for the preparation of formulations, as the physical characteristics often accompanying the different physical forms of the drug, such as hygroscopicity, flowability and/or powder compaction, can be advantageously exploited. In particular, the isomorphic forms alpha and beta are characterized by low hygroscopicity, which makes handling and storing easy.
The preparation of these crystalline forms is however rather difficult from the industrial point of view, in that they tend to transform into other crystalline forms or mixtures thereof.
There is therefore the need for a more stable crystalline hydrate form.